1. Field of Invention
This invention relates to a new and improved method and apparatus for sensing the value of a direct current (DC) component superimposed on an alternating current (AC) being measured by a watt hour meter installation and for deriving a feedback signal for nulling out the effect of the DC component on the supply AC being measured.
More particularly, the invention relates to a relatively simple and low cost peak detection method and circuit for achieving the above-described compensation of a DC component superimposed on a supply alternating current being measured in a watt hour meter installation.
2. Background Problem
It has become a practice of certain consumers of electricity who wish to lower their utility bills to resort to various measures which seriously affect the capability of a watt hour meter installation to obtain a true measure of the electricity being consumed. For example, if a diode is placed in series with the load at a consumer's dwelling or commercial installation, a direct current component is superimposed on the alternating current flowing through the load. If the particular watt hour meter installation in question is designed such that it employs a current transformer for sensing the value of the supply alternating current, the direct current component superimposed on the alternating current can result in a DC magnetizing current flowing in the current transformer which tends to saturate the core of the transformer. If saturation does occur, the current transformer no longer is capable of operating over the linear portion of its magnetization characteristic and is incapable of inducing a signal in the secondary winding of the transformer which is truely proportional to the input AC. Hence, if the current transformer is part of an electronic or solid state watt hour meter, and its core is saturated in the above manner, the watt hour meter is rendered ineffective for accurately measuring the power being consumed by the load at the installation in question.
U.S. patent application Ser. No. 33,078 filed Apr. 25, 1979 in the name of Miran Milkovic, the inventor of the present invention, for "MEANS FOR AUTOMATICALLY COMPENSATING DC MAGNETIZATION IN A TRANSFORMER" and assigned to the General Electric Company, the assignee of the present ivnention, describes a new and improved compensating means for counteracting direct current saturation to a current transformer in an electronic watt hour meter. However, the means described in application Ser. No. 33,078 requires the use of two current transformers and relatively expensive Hall elements.
The present invention describes a new and improved lower cost, peak detection method and circuit employing only a single current transformer for sensing the value of a direct current component superimposed on an alternating current supply being measured by a watt hour meter installation and for deriving a feedback signal for nulling out the effect of the DC component on the supply AC being measured.